Can somebody explain 'Eye Relief'

[Cashman]

Eye relief.

I'm not sure what it is and how it relates to the eyepieces i currently have. What effect does it have on what you see?

[John]

Eye relief is the distance in mm that your eye needs to be from the top lens of an eyepiece to see the whole field of view.

With the eyepieces in the picture the eye relief will generally be around 80% of the focal length of the eye piece, eg: the 10mm will have 8mm of eye relief. If you wear glasses when observing you will want eyepieces with longer eye relief so that you don't have to get your eye too close to the top of the eyepiece. Modern eyepiece designs can deliver this although you do have to pay a little more for that comfort.

[Cashman]

Kudos for such a clear explanation. Very well explained, thanks!